The invention involves a device and method for controlling vortex structures in a turbulent jet and therefore decreasing the noise generated by the turbulent flow.
Noise in turbulent air flows is a common problem in various areas of engineering. It is especially acute in aircraft engineering where multiple sources contribute to aircraft noise: the propulsion system (e.g. fan, jet, etc.) and the airflow along aerodynamic elements of the airframe. A comprehensive approach dealing with different noise sources is required to achieve drastic noise reduction (fan noise, airframe noise, etc.). The invented device is intended to decrease the jet noise by controlling vortex structures in a turbulent air flow.
Intensive development of aircraft engineering and the advent of a new generation of passenger aircraft have led to a dramatic increase in powerful noise sources affecting the human being. The engine exhaust jet has been the primary noise source ever since the first jet passenger airliners were built in the 1950s. The application of by-pass turbo-jet engines has helped improve the aircraft's acoustic performances but the unrelenting attempts of international organizations to impose increasingly stringent requirements on the existing and future aircraft call for more intensive efforts in terms of noise reduction. The strategic goal of the forthcoming 7th European program in aero-acoustics is to develop basic provisions and preliminary proposals for thorough improvement of the aircraft acoustic characteristics by 2010 and to devise technology solutions and well-grounded recommendations for their implementation in the aircraft industry by 2020. A balanced approach tackling different noise sources is required to achieve further reduction of noise (fan noise, airframe noise, etc.), but nevertheless the jet noise seems to remain a stumbling block in dealing with this complex task. Besides, the jet noise remains the dominating noise source and a major obstacle for further evolution of supersonic aircraft. Therefore an acute need is felt at present for putting forward new ideas including those that are based on attempts to ensure active turbulent jet noise control in addition to the development of conventional approaches to the aircraft engine noise reduction problem.
Active turbulent jet control is still at an embryo phase of development and requires improvements in the excitation system (actuators), control algorithms and measurement instruments in order to fully exploit its rich potential. All the actuators, such as glow discharge exciters, liquid injection, Helmholtz resonators and MEMs hold a lot of promise as effective flow control instruments. It is essential to find a solution that would be both technically feasible and cost-effective.
A few design solutions that might be helpful in creating actuators for active turbulent flow control are cited below:                Para-electric actuators in which the plasma of the glow or barrier discharge is used for generating the flow velocity field in the boundary layer. In these actuators, the electric field gradient causes ion acceleration that brings into play the main medium due to particle collisions (see Roth, J. R.; Sherman, D. M.; and Wilkinson, S. P. 1998: Boundary Layer Flow Control With One Atmosphere Uniform Glow Discharge Surface Plasma. AIAA-98-0328.);        Piezoelectric actuators for flow manipulation that are governed by periodical tension which makes them oscillate. The diaphragm oscillations push the air in and out through an opening (slot or hole) that connects the cavity with the environment (this flow has been called a “synthetic jet”) (see Wiltse, J. M.; and Glezer, A. 1993: Manipulation of Free Shear Flows Using Piezoelectric Actuators. J. Fluid Mech., vol. 249, pp. 261-285);        Vortex-generating jets or pulsating vortex generators that are formed by oscillating fluid impulses injected through the opening at an angle. These vortexes pick up the separated flow and thus improve the operation of aerodynamic devices (see Magill, J. C.; and McManus, K. R. 1998: Control of Dynamic Stall Using Pulsed Vortex Generator Jets. AIAA-98-0675);        Corona discharge actuators which use an ion wind generated by the charged particles acceleration near the positive and negative corona points (see Danna A. Lacoste, David Pai, and Christophe O. Laux, “Ion wind effects in a positive dc corona discharge in atmospheric pressure” AIAA 2004-354, 2005. T. R. Troutt, D. K. Mc Laughlin, “Experiments on the flow and acoustic properties of a moderate Reynolds number supersonic jet”, J. Fluid Mech., 116, 123-156 (1982)).        
However, the problem grows much more complicated if the investigation has to deal with noise control and not only flow control.